For example, a waveform observation apparatus 10 shown in FIG. 10 is used to acquire waveform data of an optical signal modulated by the fast repetition signal for observation.
This waveform observation apparatus 10 generates an optical sampling pulse Ps having a repetition cycle Ts (=N·Tx+ΔT) which is longer by a predetermined value (offset delay time) ΔT than N times (N is any integer of 1 or more, for example, 100 or 1000) the repetition cycle Tx of the waveform of a measuring target signal P and a short pulse width by means of optical sampling pulse generating means 11.
Then, the optical sampling pulse Ps generated by this optical sampling pulse generating means 11 is input to an optical sampling portion 12 together with the measuring target signal P.
A pulsed light obtained by sampling the measuring target signal P with the optical sampling pulse Ps is converted photoelectrically by the optical sampling portion 12 into an electric pulse signal Eo, which is output to an analog/digital (A/D) converter 13.
This A/D converter 13 converts the amplitude intensity of the electric pulse signal Eo to digital data and causes to store the data in a waveform data memory 14.
After a series of the waveform data stored in the waveform data memory 14 is read out by display control means 15, it is displayed as a waveform of the measuring target signal P on a display device 16.
In such a sampling type waveform observation apparatus 10, as shown in (a) of FIG. 11, the sampling timing by the optical sampling pulse Ps is shifted by a time ΔT as shown in (b) of FIG. 11 each time when the repetition waveform of the measuring target signal P is input continuously N times. Consequently, a series of waveform data obtained by sampling the waveform of the measuring target signal P under a high resolution can be observed on a screen of the display device 16 by a sampling much slower than the cycle Tx.
The waveform observation apparatus 10 of such a sampling type has been disclosed in, for example, patent document 1 mentioned below.
The observation mode required for the aforementioned waveform observation apparatus 10 includes persistence mode, averaging mode and the like.
The persistence mode is a mode which repeats an operation of sampling the measuring target signal P and displaying its acquired data on a screen of the display device for a specified time period to display a measured waveform with its incidental image, and enables changes of the waveform of the measuring target signal to be observed substantially in real time.
The averaging mode is a mode which executes averaging processing for the waveform data of plural data acquisition periods by sampling the measuring target signal P and displaying its averaged waveform, and enables waveform observation to be performed with noise component removed.
However, unless sampling at the time of sampling of the measuring target signal P is started from an identical phase position of the repetition waveform of the measuring target signal P, there occurs such an inconvenience that a displayed waveform may be shifted in a time axis each time in case of the observation mode which displays the waveform of the measuring target signal with the incidental image.
In the averaging mode, the averaging processing cannot be performed accurately and consequently, the waveform cannot be reproduced accurately, and further, the phase of the waveform and a magnitude of the fluctuation of the amplitude cannot be grasped accurately.
For this purpose, the repetition cycle of the measuring target signal or the frequency (bit rate) of the signal itself needs to be already known.
However, depending on a case, there occurs a following problem. Even if an approximate value of the repetition cycle of the waveform of a measuring target signal which is a target for observation or its frequency is known, no proper sampling cycle can be set for the waveform of a measuring target signal which is a target for observation if its accurate value is unknown, so that no desired waveform can be observed.
Further, this kind of the waveform observation apparatus has another problem that it needs an optical mixer for generating a narrow optical sampling pulse or mixing lights so that the entire apparatus including a display portion becomes complicated and expensive.
Accordingly, to solve these problems, the inventor of this application has proposed a repetition frequency detection method for the measuring target signal disclosed in patent document 2 described below as an earlier application in Japan.
Next, the repetition frequency detection method for the measuring target signal disclosed in the patent document 2 will be described.
The measuring target signal is assumed to be a sine wave of a single frequency Fx and the frequency component of a signal Sx obtained by sampling this with a temporary sampling frequency Fs′ will be considered.
If the sampling pulse is an ideal pulse having an infinitely small width, its frequency component has respective spectrums (n=0, 1, 2, . . . ) of the frequency n·Fs′ as shown in FIG. 12.
Thus, components of a difference and sum of the frequency Fx of the measuring target signal and each frequency n·Fs′ are contained in the signal Sx obtained by sampling with this sampling pulse.
Of these, a component having the lowest frequency is a difference frequency of a spectrum component of the frequency n·Fs′ most proximate to the frequency Fx or a difference frequency of a spectrum component of a frequency (n+1)·Fs′ as shown in (a) and (b) of FIG. 13 and that difference frequency Fh′ can be expressed as follows.Fh′=mod[Fx, Fs′] . . . (case of mod[Fx, Fs′]≦Fs′/2)Fh′=(Fs′/2)−mod[Fx, Fs′] . . . (case of mod[Fx, Fs]>Fs/2)where a symbol mod[A, B] indicates a remainder when A is divided by B.
Because this difference frequency Fh′ is Fs′/2 at maximum, it can be extracted simply using a low pass filter having a band upper limit of Fs′/2.
Here, a change δFh of the difference frequency Fh′ accompanying a minute change δFs of the temporary sampling frequency Fs′ is given in a following equation of differentiating the difference frequency Fh′ with respect to the frequency Fs′.δFh/δFs=−quotient [Fx, Fs′] . . . (case of 0<mod[Fx, Fx′]<Fs′/2)δFh/δFs=1+quotient [Fx, Fs′] . . . (case of mod[Fx, Fx′]>Fs′/2)where the symbol quotient [A, B] indicates an integer quotient when A is divided by B.
As a result of this, the frequency Fx of the measuring target signal can be obtained by the following equation according to a relation between the quotient and the remainder, mod[Fx, Fs′]=Fx−Fx′·quotient [Fx, Fs′],Fx=Fh′−Fs′·δFh/δFs (case of 0>δFh)Fx=−Fh′+Fs′·δFh/δFs (case of 0<δFh)FIG. 14 is a flow chart showing an example of the procedure of the repetition frequency detecting method for the measuring target signal.
First, a measuring target signal is sampled with the temporary sampling frequency Fs′ (step S1) and of signals obtained by that sampling, the frequency Fh′ of a specific signal which appears in a band ½ of the temporary sampling frequency Fs′ is detected (step S2).
Then, the temporary sampling frequency Fs′ is changed by a minute amount ΔFs (for example, 1 Hz) (step S3) and the frequency change amount ΔFs of the specific signal is detected (step S4).
By substituting the temporary sampling frequency Fs′, its frequency change amount ΔFs, the frequency Fh′ of the specific signal and its frequency change amount ΔFh into the following equation (1), the repetition frequency Fx of the measuring target signal is calculated (step S5);Fx=Fh′−Fs′·ΔFh/ΔFs (case of 0>ΔFh)Fx=−Fh′+Fs′·ΔFh/ΔFs (case of 0<ΔFh)   (1)
In a system for acquiring and observing the waveform information, this frequency detecting processing is carried out on a measuring target signal and then, by setting a regular sampling frequency Fs corresponding to the obtained frequency Fx, acquisition and observation for the waveform information of the measuring target signal can be carried out accurately.
Further, the patent document 2 has disclosed a waveform observation system containing a sampling apparatus to which the repetition frequency detecting method for the measuring target signal is applied.
FIG. 15 shows the structure of the waveform observation system 20 containing the sampling apparatus to which the repetition frequency detecting method for the aforementioned measuring target signal is applied.
This waveform observation system 20 is constituted of a sampling apparatus 21 and a digital oscilloscope 60.
In the sampling apparatus 21, a measuring target signal P input through an input terminal 21a is sampled by an optical sampling portion 26 according to a sampling pulse which is a narrow optical pulse, generated from the sampling pulse generating portion 25 based on a clock signal C generated by the signal generating portion 24 so as to obtain a pulse signal Eo as its waveform information.
The digital oscilloscope 60 stores and displays the waveform information obtained by the sampling apparatus 21.
This sampling apparatus 21 has a manual setting mode which is specified when the repetition cycle of an observation target waveform is known accurately and an automatic setting mode which is specified when the repetition cycle of the observation target waveform is unknown or only its approximate value is known. The manual setting mode and automatic setting mode can be specified selectively by an operation of an operating portion, not shown.
In the meantime, a clock signal C and a trigger signal G generated by the signal generating portion 24 can be output through a clock output terminal 21b and a trigger output terminal 21d. 
Likewise, it is so configured that the pulse signal Eo from the optical sampling portion 26 can be output outside through a sampling signal output terminal 21c. 
The output terminals 21b to 21d of this sampling apparatus 21 are connected to an external clock input terminal 60a, first channel input terminal 60b and second channel input terminal 60c of the digital oscilloscope 60, respectively.
The digital oscilloscope 60 has an external clock synchronous function which executes A/D conversion processing for a signal input through the channels 60b, 60c synchronously with a clock signal input to the external clock input terminal 60a, an external trigger function which stores data obtained by the A/D conversion processing as waveform data for each channel until a specified time (depending on a display width of the time axis, the quantity of display points and the like) elapses since a timing at which the voltage of an input signal of an arbitrarily specified channel input terminal or trigger input terminal exceeds an arbitrarily set threshold in a predetermined direction, and a waveform display function which displays the stored waveform data on the time axis. As this waveform display mode, it is so configured that any one of the aforementioned persistence display mode and the averaging display mode can be displayed arbitrarily.
Next, an operation of the waveform observation system 20 will be described.
First, a measuring target signal P which is a substantially rectangular wave having a duty ratio of 50% is input to the input terminal 21a as shown in (a) of FIG. 16 and information corresponding to a substantial repetition cycle Tx′ (frequency Fx′) of the waveform and an offset delay time ΔT of the sampling is specified by a parameter specifying portion 22 and then, the automatic setting mode is specified by an operating portion, not shown.
The arithmetic operation portion 23 calculates the temporary sampling frequency Fs′ and trigger frequency Fg′ based on the specified substantial repetition frequency Fx′ and offset delay time ΔT and sets them in the signal generating portion 24.
In the meantime, if the automatic setting mode is specified with the repetition cycle Tx′ (frequency Fx′) not specified, the arithmetic operation portion 23 executes arithmetic operation according to a specified value, for example, with 10 GHz set as the repetition frequency Fx′.
Thus, the clock signal C of the temporary frequency Fs′ is output from the signal generating portion 24.
The optical sampling portion 26 executes sampling for the measuring target signal P according to the temporary sampling frequency Fs′ and a pulse signal Eo obtained by the sampling is input to a specific signal frequency detecting portion 27.
Of the frequency components contained in the pulse signal Eo obtained by that sampling, the specific signal frequency detecting portion 27 regards a frequency component having the highest level which appears in a band ½ of the temporary sampling frequency Fs′ as a specific signal and detects its frequency Fh′.
As regards the waveform of this optical signal, the spectrum of the optical sampling pulse Ps for use in sampling appears every frequency Fs′ as shown in FIG. 17 and the spectrum of the waveform of the measuring target signal P appears every frequency Fx while the level is decreased as its order is increased.
Accordingly, the specific signal frequency detecting portion 27 obtains a difference frequency Fh′ between the frequency Fx at the lowest order and a temporary sampling frequency component n·Fs′ most proximate to that frequency Fx as the frequency of the specific signal and outputs it to a repetition frequency calculating portion 28.
When the frequency Fh′ of the specific signal concerning the temporary sampling frequency Fs′ is obtained, the repetition frequency calculating portion 28 stores this frequency Fh′ and instructs the signal generating portion 24 to change the temporary sampling frequency Fs′ by a minute amount (for example, 1 Hz).
The temporary sampling frequency Fs′ to the measuring target signal P is changed by only the minute amount ΔFs by the signal generating portion 24 which receives this instruction and accompanied by this change, the frequency of the specific signal detected by the specific signal frequency detecting portion 27 is changed by only ΔFh. The repetition frequency Fx of the waveform of the optical signal is calculated by this change amount according to the following equation and set in the arithmetic operation portion 23.Fx=Fh′−Fs′·ΔFh/ΔFs 
The arithmetic operation 23 calculates a regular sampling frequency Fs and trigger frequency Fg corresponding to the input signal accurately based on the accurate repetition frequency Fx calculated by the repetition frequency calculating portion 28 and sets them in the signal generating portion 24.
As a result, a clock signal C having the same cycle as N·Tx+ΔT and optical sampling pulse Ps are generated with respect to the repetition cycle Tx having the waveform of the measuring target signal P as shown in (b) and (c) of FIG. 16.
Then, the measuring target signal P is sampled by the optical sampling pulse Ps and a pulse signal Eo obtained by that sampling is input to the first channel input terminal 60b of the digital oscilloscope 60 from the optical sampling portion 26 through the sampling signal output terminal 21c as shown in (d) of FIG. 16.
A trigger signal G having the same cycle as that of the waveform of an envelope curve which connects the peaks of the pulse signal Eo as shown in (b) of FIG. 18 is generated from the signal generating portion 24 and input to the second channel input terminal 60c of the digital oscilloscope 60 through the trigger output terminal 21D.
Furthermore, (a) of FIG. 18 shows the waveform shown in (d) of FIG. 16 such that its time axis is contracted.
The digital oscilloscope 60 executes the A/D conversion processing for the pulse signal Eo synchronously with the clock signal C, outputs data of the envelope curve which connects the peak points of the pulse signal Eo successively in the form of the optical signal waveform data and starts acquisition of the waveform data at a timing when the trigger signal G exceeds a trigger level in a predetermined direction.
As a result, a waveform of the optical signal P is displayed with points at an interval ΔT in terms of offset delay time on the screen of the digital oscilloscope 60 as shown in, for example, FIG. 19 in the form of an incidental image.
The digital oscilloscope 60 starts acquisition of the waveform data at each timing when the trigger signal G exceeds a trigger level in a predetermined direction and displays the waveform by updating. Because the sampling frequency and trigger frequency of the sampling apparatus 20 correspond to the repetition frequency of the waveform of the optical signal P to be input accurately, the position of the waveform always displayed is never deflected, thereby stable waveform observation being achieved.
Patent document 1: Jpn. Pat. Appln. KOKAI Publication No. 2002-071725
Patent document 2: Jpn. Pat. Appln. KOKAI Publication No. 2006-3327